Insulated gate bipolar transistor (IGBT) modules are widely used in inductive (hard) switching voltage source power electronic converters such as drives, switched-mode power supplies or solid state transformers.
Minimizing switching losses of the IGBT, sticking the current and voltage conditions to the save operating area (SOA), e.g. limiting the turn-off overvoltage and the peak reverse recovery current during turn-on, and restricting electromagnetic interference (EMI) can be a challenging task of the IGBT's gate drive. Setting the collector current slope diC/dt at turn-on can enable the peak reverse recovery current to be limited, and diC/dt during turn-off can define the overvoltage that results due to the voltage drop across the total commutation loop inductance Lσ. To provide electromagnetic compatibility (EMC), as the case may be, the collector-emitter voltage slope dvCE/dt and diC/dt have to be restricted to specified values.
An independent current and voltage slope control individually for turn-on and turn-off allows the gate drive to optimally switch the IGBT in all operating points with minimal switching losses while sticking to the SOA and providing EMC. In addition, if IGBT modules are directly connected in series or in parallel, the control of dvCE/dt or diC/dt enables a symmetrical voltage or current sharing.
A simple and common way of adjusting the switching speed of IGBTs is to insert additional passive components into the circuit. Additional gate resistance RG reduces the gate current and therefore also both current and voltage slopes, extra Miller capacitance CGC lowers dvCE/dt and added gate-emitter capacitance CGe slows down diC/dt. This approach can lead either to excessive switching losses or to increased delays and gate driving losses due to the larger amount of gate charge that is specified.
To avoid additional gate driving losses, a feed-forward gate voltage shape generator can be used to adjust the diC/dt. An example of this technique is disclosed in P. J. Grbovic, “An IGBT gate driver for feed-forward control of turn-on losses and reverse recovery current,” IEEE Transactions on Power Electronics, vol. 23, no. 2, pp. 643-652, March 2008. However, in this approach the controllability of the voltage slope is small.
Further possibilities to influence the gate current during the switching transients are for example switchable or adjustable gate resistor(s), current sources/sinks or gate voltages. As the implementation of such gate drives with adjustable output stage has to ensure an operation in the SOA, e.g. limited diC/dt and dvCE/dt, for all operating conditions (varied Tj, iC, vCE), for most of the operating points the desired optimal current and voltage slopes are not achieved leading to increased switching losses. In addition, the system state of the semiconductor, e.g. the transition from the current to the voltage transients and vice versa, should be detected most accurately in an additional complex circuit, to be able to independently adjust diC/dt and dvCE/dt.
Missing compensation of the IGBT's non-linearities and dependencies on the operating point is a further and main drawback of all these open-loop control topologies. The IGBT's transconductance gm in fact varies with the gate voltage VGe and the junction temperature Tj as well as both capacitance values CGe and CGC depend on the applied voltage, such as the Miller capacitance. With an open-loop approach, accurately defined and constant current and voltage slopes can therefore not be obtained. For that reason, topologies with feedback are applied to achieve more precise control.
Best performance with regard to analog control bandwidth is achieved by means of diC/dt and dvCE/dt control topologies due to simple and high-bandwidth measurement circuits, easy to generate constant reference value(s) and simple control amplifier stages. Different implementations of only diC/dt control or individual solutions for current or voltage slope control during turn-on or turn-off has been discussed in S. Park and T. M. Jahns, “Flexible dv/dt and di/dt control method for insulated gate power switches,” IEEE Transactions on Industry Applications, vol. 39, no. 3, pp. 657-664, 2003.
A complete solution of turn-on and turn-off diC/dt and dvCE/dt control was disclosed in C. Dorlemann and J. Melbert, “New IGBT driver with independent dv/dt- and di/dt-feedback control for optimized switching behavior,” Proc. of the 2nd Int. Conf. on Integrated Power Electronic Systems (CIPS), pp. 107-114, 2002. Due to the implementation with a large amount of bipolar transistors and an active detection and selection of the control loop, the performance was limited to 200 A/μs and 1 kV/μs.